Solvent extraction reagents containing the diphosphonic acid group exhibit extraordinary affinity for tri-, tetra-, and hexavalent actinides. Use of these reagents has been proposed for actinide separation and preconcentration procedures. A new extractant, P,P'-di(2-ethylhexyl) benzene- 1,2-diphosphonic acid (H2DEH[1,2-BzDP]), has been synthesized. Its metal extraction characteristics are reported and compared to the previously synthesized P,P'-di(2-ethylhexyl) methylene-(H2DEH[MDP]), ethylene-(H2DEH [EDP]), and butylene-(H2DEH[BuDP]) diphosphonic acids. Different degrees of aggregation and different extraction mechanisms and stoichiometries have been reported for these ligands.In this work, thermodynamic parameters associated with the extraction of selected metal ions by these ligands were measured to provide further insight into the mechanism of the extraction processes involved. Knowledge of the enthalpy, entropy, and free-energy changes, in combination with other fundamental principles governing extraction of metal ions by these ligands will increase their usefulness in liquid-liquid extraction. Thermodynamic parameters of the extraction process were determined by the temperature dependence of the distribution ratio in the temperature range of 25 to 60degreesC. Neither the aggregation state of the free ligands nor the extraction stoichiometries at trace metal concentration level change in the whole range of temperatures investigated. The effect of temperature on the extraction of Am-241, U-233, and Sr-85 from HNO3 solutions by o-xylene solutions of the extractants are reported.